EEG and MEG Flashcards
Define EEG (encetoencephalography)
A way of measuring small voltage changes at the scalp caused by electrical currents from neurons. There can be between 32 and 128 electrodes placed on the scalp to measure these (sometimes less in clinical settings).
What is the typical size of an EEG signal?
A few µV
Define MEG (magnetoencephalography)
A way of detecting small magnetic fields induced by the same electric currents as EEG (from neurons). Superconducting Quantum Interference Devices (SQUIDs) are used to measure these. An MEG helmet has ~300 sensors that are radially oriented.
What is the typical size of an MEG signal?
~ 10⁻¹⁵ T
Both EEG and MEG are based on ________ activity and are a direct measure of ______ ________.
Electrical
Brain activity
Describe the temporal resolution of EEG and MEG
Both have excellent temporal resolution: < 1ms
Describe the temporal resolution of fMRI
Very poor temporal resolution: ~5 s
Describe the spatial resolution of EEG and MEG
Both have poor spatial resolution.
EEG: ~20-30 mm
MEG: ~5 mm
Describe the spatial resolution of fMRI
Very good spatial resolution: ~1 mm
Compare the cost of EEG, MEG, and fMRI
EEG is the cheapest at £20,000.
fMRI and MEG are more expensive at £1,000,000.
What are the 3 main categories of neuro-electrical effects recorded by EEG and MEG?
- Spontaneous rhythms
- Evoked effects
- Induced effects
What are spontaneous rhythms?
Naturally occurring rhythms that don’t require external stimulation.
What is the alpha rhythm?
The most easily detected rhythm at ~10 Hz in frequency. It is thought to be a measure of the level of cortical inhibition and is most prominent for resting, awake subjects with their eyes closed.
What is the frequency range of the alpha rhythm?
8-13 Hz
What is the frequency range of the delta rhythm?
< 4 Hz
What is the frequency range of the theta rhythm?
4-8 Hz
What is the frequency range of the beta rhythm?
13-30 Hz
What is the frequency range of the gamma rhythm?
30-100 Hz
What are evoked effects?
A response to an external stimulus. It is averaged across all stimulus repeats because these effects are time-locked and phase-locked to the stimulus and occur shortly after the stimulus onset.
What are induced effects?
Stimulus-induced changes in spontaneous rhythms. These are NOT phase-locked to the stimulus onset or offset.
How are induced effects recorded so that the result is observable?
The signal is squared to get the power average over repeats because the effects are not phase-locked so averaging across trials without any moderation loses these effects from the signal.
Describe the composition of a neuron
A single neuron consists of the cell body, the dendrites, and an axon.
What are the two types of neurons?
- Pyramidal neurons
- Stellate neurons
Describe pyramidal neurons
Neurons whose dendrites are parallel to each other and are perpendicular to the cortical surface, causing current flow to be normal to the cortical surface.
Describe stellate neurons
Neurones whose dendrites are symmetrically distributed so there is isotropic current flow and little EM field at a distance.
Which type of neuron generates fields that are detectable in EEG and MEG?
Pyramidal neurons
Describe the process of generating a post-synaptic current in a neuron
- Initially (at resting state) there is an ion imbalance across the neuron membrane that gives a resting potential of -70 mV.
- When an action potential arrives at a neuron from a pre-synaptic cell, it causes neurotransmitters to be released across the synaptic cleft.
- Ion channels open in the post-synaptic cell, allowing sodium ions (Na+) to rush into the cell and travel down the dendrites.
- This generates an intracellular current inside the neuron and an equal volume current in the surrounding tissue.
The intracellular current is the post-synaptic current.
Does EEG detect intracellular (primary) or volume current?
Volume current
Does MEG detect intracellular (primary) or volume current?
Primary current
The post-synaptic current falls off as ____.
1/r²
Post-synaptic potentials change the _______ of the soma.
Voltage
How high does the post-synaptic potential have to be to generate an action potential that is sent down the axon?
It must reach -40 mV from -70 mV
How fast is an action potential?
~1 ms
What are the 2 parts of an action potential?
- Leading edge of depolarisation
- Trailing edge of repolarisation
An action potential falls off as ____
1/r³
Why do MEG and EEG measure the post-synaptic currents from pyramidal neurons rather than action potentials?
Action potentials fall off faster with distance so aren’t picked up.
Give the equation for the current dipole strength from a dendrite
|Q| = Iλ
|Q| = curent dipole strength
I = primary current
λ = current decay distance
What is the typical current decay distance along a dendrite?
~ 0.15 nm
Give the equation for neuron resistance per unit length
r_s = resistance per unit length
σ = conductivity of neuron
d = neuron diameter
State one cause of cancelling fields from brain signals
Pyramidal cells are organised radial to the cortical surface so folds in the cortex can cause cancelling fields.
What is a scalp topography (field) map?
A map of the EM fields at several points over the scalp.
Field maps produced by MEG and EEG are _______ to each other (right-hand rule).
Orthogonal
State 3 applications of EEG
- Clinical uses (e.g. epilepsy, sleep staging, and sleep disorders) as cheap and readily available
- Research
- Brain-computer interfaces
State the current application of MEG
- Research into epilepsy and schizophrenia (still on patients but not in the NHS)
